Generally, an elevator car is supported by a wire rope which attaches at a first end to an elevator car, passes over a drive sheave, and attaches at the other end to a counterweight. The car is raised or lowered via a traction force developed between the wire rope and the drive sheave.
Generally, the rope is disposed on a drive sheave in one of three ways. One way, generally used in low-speed applications (i.e., low-rise buildings), utilizes a "single-wrap" in which the rope contacts about 180.degree. of a groove within the periphery of the drive sheave. To improve the traction between the rope and the groove, the groove is often shaped, e.g. with a V-cut or undercut, or provided with a plastic insert.
A second way, used in higher-speed applications (i.e., high-rise buildings), utilizes a "double-wrap" in which the rope contacts approximately 90.degree.-180.degree. of first and second parallel grooves provided within the periphery of the drive sheave for increased traction. After one wrap of from 90.degree.-180.degree. in the first groove, an idler sheave displaces the rope for a second wrap (180.degree. ) in the second groove.
The third way utilizes a "triple-wrap" arrangement. Such an arrangement is shown in U.S. Pat. No. 4,753,322 to Yasuda and is owned by the assignee herein. The reference to Yasuda is hereby incorporated by reference herein. According to Yasuda, the rope is helically wrapped for at least 360.degree. within a groove disposed in the periphery of the drive sheave. The groove has a width of at least twice the diameter of the rope, and a depth of less than half the diameter of the rope to minimize friction between the rope and the edges of the groove. The groove has a flat bottom to facilitate the rope's movement from one side of the groove to the other as the sheave drives the rope.
Ideally, the rope is helically wrapped around 540.degree. of the sheave within the groove. During rotation of the sheave, the rope enters one side portion of the groove at a "pay-on" point, wraps 180.degree. on the one side of the groove over an upper half of the sheave, continues to wrap another 180.degree. around the lower half of the sheave, continues to wrap 180.degree. on the other side portion of the groove over the upper half of the sheave and exits from the groove at a "pay-off" point. The rope, at the pay-on point tends to push the second wrap towards the other side of the groove.